Intraluminally implantable frames are being implanted in increasing numbers to treat a variety of conditions, and are coming into greater use in a variety of fields. Frames implanted in vessels, ducts or channels of the human body can form part of a valve to regulate fluid flow within a body lumen or as scaffolding to maintain the patency of the vessel, duct or channel lumen. Implantable frames can also support a valve or valve leaflets for regulating fluid flow within a body lumen or for dilating a body lumen. One or more flexible valve leaflets can be attached to an implantable frame to form a medical device useful as an artificial valve. A variety of other implantable prostheses, such as stents, grafts and the like, also comprise an implantable frame placed within the body to improve the function of a body lumen.
The venous system includes a series of valves that function to assist the flow of blood returning to the heart. These natural valves are particularly important in the lower extremities to prevent blood from pooling in the lower legs and feet during situations, such as standing or sitting, when the weight of the column of blood in the vein can act to prevent positive blood flow toward the heart. This condition, commonly known as chronic venous insufficiency, is primarily found in individuals in which gradual dilation of the veins, thrombotic events, or other conditions prevent the leaflets of the native valves from closing properly. The failure of native valves to properly close can worsen, leading to significant leakage of retrograde flow such that the valve can become incompetent. Chronic venous insufficiency is a condition in which the symptoms can progress from painful edema and unsightly spider or varicose veins to skin ulcerations. Elevation of the feet and compression stocking can relieve symptoms, but do not treat the underlying disease. Untreated, the disease can impact the ability of individuals to perform in the workplace or maintain their normal lifestyle.
One promising approach to treating venous valve insufficiency includes the implantation of self-expanding or radially-expandable artificial valves that can be placed using minimally invasive techniques. One common problem evident from early prosthetic valves is the formation of thrombus around the base of the leaflets, possibly due at least in part to blood pooling in that region. In a natural valve, the leaflets are typically located within a sinus or enlargement in the vein. The pockets formed between the leaflets and the walls of the sinus can create vortices of flowing blood that help flush the pocket and prevent blood from stagnating and causing thrombosis around the valve leaflets, which can interfere with the function of the valve. Stagnating blood may restrict oxygen from reaching the endothelium covering the valve cusps, leading to hypoxia of the tissues which may explain increased thrombus formation typical in that location. Expandable-frame valve prostheses are typically cylindrical in shape and lack an artificial sinus or pocket space that is sufficient for simulating these natural blood flow patterns.
What is needed is an intraluminally-placed medical device, such as an artificial valve or stent, that is configured to create more desirable flow patterns around a valve within a body, for instance to circulate the blood or bodily fluids and reduce the likelihood of stagnation and the potential clinical problems that may result. Implantable devices that have structural adaptations resulting in the formation of sinus regions within a body vessel can create beneficial fluid flow conditions such as more turbulent flow, increased velocity of flow, larger and/or more numerous vortices, other factors, or a combination of the above that can mitigate the incidence of thrombosis formation near the implantable medical device.